Minimize unnecessary message deserialization

[DonJayamanne]

Part of #5662
Simple change, deserialization is a little slower, hence using plain string operations over deserialization.

[rchiodo]

This is a pretty big assumption. How do you know that all messages for a widget will fit into these subcategories? And what about when we start supporting synapse?

What's the perf improvement of this change? Seems risky to me.

[rchiodo]

Misread the section below. THis is just like the previous change.

[codecov-commenter]

Codecov Report

Merging #6946 (7946ea8) into main (13cf495) will increase coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #6946   +/-   ##
=====================================
  Coverage     65%     65%           
=====================================
  Files        410     410           
  Lines      28528   28528           
  Branches    4263    4263           
=====================================
+ Hits       18584   18628   +44     
+ Misses      8316    8272   -44     
  Partials    1628    1628           

Impacted Files	Coverage Δ
...ent/datascience/jupyter/pythonVariableRequester.ts	65% <0%> (-2%)	⬇️
...client/datascience/kernel-launcher/kernelDaemon.ts	53% <0%> (-2%)	⬇️
...ient/datascience/editor-integration/codewatcher.ts	68% <0%> (-1%)	⬇️
src/client/datascience/jupyter/jupyterNotebook.ts	66% <0%> (ø)
src/client/datascience/jupyter/kernels/helpers.ts	73% <0%> (+<1%)	⬆️
...t/datascience/notebook/vscodeNotebookController.ts	80% <0%> (+<1%)	⬆️
...lient/datascience/jupyter/kernels/cellExecution.ts	71% <0%> (+<1%)	⬆️
src/client/datascience/telemetry/telemetry.ts	73% <0%> (+1%)	⬆️
src/client/datascience/baseJupyterSession.ts	66% <0%> (+1%)	⬆️
src/client/datascience/notebook/notebookEditor.ts	41% <0%> (+1%)	⬆️
... and 10 more